The present application relates to the use of phasor measurements in electrical power system state estimation. It finds particular application to the integration of voltage and current phasors in the state estimation process.
Power system state information plays an important role in the operation and analysis of electrical power systems. Indeed, state information can be used for a number of purposes, including system security and contingency analyses, system optimization, and even the creation and operation of markets for electrical energy.
Traditionally, state estimators have performed an indirect estimation of the power system state (i.e., voltage magnitude and angles at the network buses, tap positions of tap changing transformers, etc.) using non-phasor measurements such as power flows and voltage and current magnitudes, to name a few. More recently, however, phasor measurement units (PMUs) have been developed. PMUs provide time-stamped measurements of voltage and current phasors at various locations in the power system. This additional information can be used to improve the network state estimation.
One of the difficulties inherent in providing phase angle measurements in a widely dispersed system, however, is the need to synchronize the measurements to a common reference frame. To this end, PMUs have provided phasor measurements which are synchronized based on time signals from the global positioning system (GPS). Moreover, the Institute of Electrical and Electronics Engineers (IEEE) Synchrophasor standard defines a phasor's instantaneous phase angle in relation to Universal Time Coordinated (UTC), hence fixing the frame of reference for the PMU phasor measurements.
Due to the nature of power system networks, where the power flows on branches on the network are a function of the differences of the voltage phase angles across the branches, a reference or slack bus is chosen in order to render the state estimation problem solvable. Thus, a common reference angle, known as the slack bus angle, has been selected for the state estimation process. While it is generally possible to assign the slack bus angle to an arbitrary value, it is typically set to zero.
As will be appreciated, it is necessary to reconcile the phasor measurement and state estimator reference frames. This can be accomplished by requiring that a reference PMU be placed at the location of the slack bus and computing the angles of the various PMUs with respect to the reference PMU. Unfortunately, however, such an arrangement renders the system vulnerable to the loss of the signal provided by the reference PMU, for example due to an outage at a measurement location, a failure of the reference PMU, or a communication failure. While it is possible to compensate for such a loss, the process for doing so is relatively time-consuming and complex.